Recently, since the manufacturing technology for the LCD has gradually become mature, and, meanwhile, all the main manufacturers in the world have invested a lot of time and money in the relevant studies and used large-scale instruments, the qualities of the LCD have been substantially improved. Since the liquid crystal television has become the leading role in the tide of applying the digital electric instruments, it is the most expected and has the most market potential in the applications of the LCD.
The LCD is not a displaying system with self-illumination, so that it always needs an additional light source device, such as the backlight module. In general, the backlight module includes a plurality of cold cathode lamps, so that the backlight module must have the thick diffusers and the thickness thereof is not easily reduced. In addition, since the conventional backlight module has a plurality of lamps, its operation temperature is always high. Besides, the decay rate of each lamp is always accelerated due to the long operation period under the high operation temperature and high illumination. However, the most serious problem is that the decay rate of each lamp is different from others, so that, after a long period of operation, the illumination of the backlight module will become uneven. In order to solve the above possible drawback, the backlight module with flat lamps was provided.
Please refer to FIG. 1, which shows a diagram of the conventional cold cathode flat fluorescent lamp (CCFFL). As shown in FIG. 1, the cold cathode flat fluorescent lamp 10 includes the upper glass plate 11, the lower glass plate 12, the metal electrodes 13 and 14, the insert gas molecules (not shown), and the phosphor powder (not shown). The metal electrodes 13 and 14 can be provided on the external of the cold cathode flat fluorescent lamp to form an external electrode cold cathode flat fluorescent lamp.
The light emitting principle of the cold cathode flat fluorescent lamp 10 is described as follows. A voltage difference is provided between the metal electrodes 13 and 14 so as to make the metal electrodes 13 and 14 absorb or emit electrons. The electrons will hit the insert gas molecules to excite them into the plasma. When the insert gas molecules are transformed from their excited states back to the stable states, the ultraviolet will be emitted therefrom. After that, the emitted ultraviolet will excite the phosphor powder to generate the visible light.
In traditional, the cold cathode flat fluorescent lamp is driven by a single-pulse voltage, such as that shown in FIG. 2. Please refer to FIG. 2, which is a wave diagram of the conventional single-pulse voltage and the current of the cold cathode flat fluorescent lamp.
As shown in FIG. 2, since only a single voltage is applied to drive the cold cathode flat fluorescent lamp, only a single light will be emitted therefrom. The driving method with a single-pulse voltage has the following defects.
(1). Since the used driving voltage is a single-pulse voltage with constant polarity, the cations and anions separated from the insert gas molecules will attach and accumulate on the metal electrodes to form the accumulated wall charges. The existence of the accumulated wall charges results in that the following driving voltage must be increased so as to obtain the same driving effect. However, the higher driving voltage will cause a higher operation temperature for the cold cathode flat fluorescent lamp and the electric arc for the cold cathode flat fluorescent lamp will become unstable.
(2). Since the used driving voltage of the cold cathode flat fluorescent lamp is usually a high voltage (which is usually higher than 2 kV), the electromagnetic interference (EMI) is always serious. However, the serious EMI of the cold cathode flat fluorescent lamp may result in that the backlight module assembled with the cold cathode flat fluorescent lamp cannot be compliable to the regulations of EMI/EMC thereof.
In order to overcome the defects of the conventional CCFFL, it's an object of the present invention to increase the illumination effect of the CCFFL and to reduce the EMI resulting from the high driving voltage.
In addition, it's another object of the present invention to reduce the situation that the cations and anions always accumulate on the metal electrodes resulting from the driving method with single-pulse voltage for the conventional cold cathode flat fluorescent lamp.